THE LOCALISATION OF BEEAKS AND FAULTS. 



529 



the nearest position to balance, and then the single pointer to 

 get the balance exactly. Balance is obtained when both 

 battery circuits are closed by the keys and the galvanometer 

 remains at zero. 



In reading the slides the following should be observed : — 

 Bead first from the double pointer, taking the figure from 

 the pointer nearest zero. Multiply this by 100 and add the 

 reading on the single pointer. 



For example, say the standard cell is balanced when the 

 pointer nearest zero (o^ the double pointer) is at 2 and the 

 single pointer is at 74. Also that they are respectively at 87 

 and 8 when the battery is balanced in place of the standard 



Main Battery 



100,00 ohms ' 



'' VNAAAAAA^ ( \ 



Galvanometer^^_^y ] 



Standard Cell 



Fig. 299. 



cell. The readings are then respectively 274 and 8,708, and if 

 the standard cell is 1*4# volts the E.M.F. of the battery is 



,8708 



l-4x' 



274 



=44-5 volts. 



This method is absolutely accurate, but it requires an inde- 

 pendent main battery on the slides, which may not always be 

 available. 



2. The second method is without an independent battery. 

 In its place is put the battery whose E.M.F. is to be tested, 

 that is direct on the slides, between terminals and 101 on the 

 large coils A B. The connections are otherwise the same, and 

 one balance only is required, namely, with the standard cell 

 connected, as in Fig. 298. The battery to be tested by this 

 particular method is to be assumed as in the position of " main 

 battery " in the diagram. 



